1. Field of the Invention
The present invention relates generally to the recovery of metal values from ores. More particularly, the present invention relates to the recovery of metal values by the leaching of ores. Still more particularly, the present invention relates to an improvement in the treatment of gold/silver ores by the method of heap-leaching in which the metal values are recovered as a cyanide complex.
2. Description of the Prior Art
The recovery of gold and silver by heap-leaching low grade ores and tailings from other recovery processes is well known. In general, the procedure comprises spraying, trickling or pouring on, or otherwise applying to, a pile of low grade ore or tailings an aqueous alkali cyanide solution, e.g., aqueous calcium, sodium or potassium cyanide, so as to cause the solution to permeate and percolate through the pile thereby extracting the metal values as a cyanide complex. The resultant metal cyanide complex-bearing liquor is recovered from the bottom of the pile, of which there is usually a series in an extraction facility, and routed to a recovery system for separating the metal cyanide complex by adsorption on a column of activated carbon, usually coconut shell carbon. The metal cyanide complex-bearing carbon particles are then further treated, as by electrowinning, to separate and recover the elemental metal value.
Various factors can affect the economics of heap leaching, a principal one being the physical nature of the crushed ore being treated. For instance, the presence of a high content of slimes or fines in a highly clayey ore can, in the presence of the aqueous leach solution, result in the swelling of the fines and the filling of the interstices of the ore particles, or even a coating of the particle surfaces. As a consequence, the porosity of the ore particles is reduced thereby preventing effective percolation of the leach solution with corresponding decreased leach solution percolation rate and decreased metal recovery rate.
To minimize the effect of slimes or fines, various treatments, e.g. flotation, have been practice on the ore prior to being heap leached in order to reduce the fines content. Various means have also been use during the heap-leaching procedure itself in an effort to increase the rate and quantity of metal recovery. CA92(16):132551q, for example, reports an increase in the percolation rate in the heap leaching of gold/silver ores by using a cyanide leach solution containing a flocculant such as polyethylene oxide. On the other hand, the use of a nonionic ethylene/propylene oxide-based surfactant in a gold/silver cyanide heap leaching solution, as reported in "Gold and Silver Heap and Dump Leaching Practice", pp. 41-49, Soc. Mining Eng., Proc. Fall 1983 SME Meeting, showed no affect on percolation rate, metal recovery rate or reagent consumption. The use of the surfactant, moreover, resulted in a drastic drop in the carbon efficiency of the subsequent metal recovery system.